tests/docker: cleanup non-verbose output

Even with --quiet docker will spam the sha256 to the console. Avoid
this by redirecting stdout. While we are at it fix the name we echo
which was broken during 0b1a649047 (tests/docker: use direct RUNC call
to build containers).

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230829161528.2707696-3-alex.bennee@linaro.org>

tests/docker/Makefile.include

@@ -47,8 +47,8 @@ docker-image-%: $(DOCKER_FILES_DIR)/%.docker
 		$(if $(NOUSER),,			\
 			--build-arg USER=$(USER)	\
 			--build-arg UID=$(UID))		\
-		-t qemu/$* - < $<, 			\
-		"BUILD", $1)
+		-t qemu/$* - < $< $(if $V,,> /dev/null),\
+		"BUILD", $*)
 
 # Special rule for debootstraped binfmt linux-user images
 docker-binfmt-image-debian-%: $(DOCKER_FILES_DIR)/debian-bootstrap.docker